[llvm] [InstCombinePHI] Enhance PHI CSE to remove redundant phis (PR #163453)

[Yingwei Zheng via llvm-commits]

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@@ -1621,11 +1621,101 @@ Instruction *InstCombinerImpl::visitPHINode(PHINode &PN) {
     // Note that even though we've just canonicalized this PHI, due to the
     // worklist visitation order, there are no guarantess that *every* PHI
     // has been canonicalized, so we can't just compare operands ranges.
-    if (!PN.isIdenticalToWhenDefined(&IdenticalPN))
-      continue;
-    // Just use that PHI instead then.
-    ++NumPHICSEs;
-    return replaceInstUsesWith(PN, &IdenticalPN);
+    if (PN.isIdenticalToWhenDefined(&IdenticalPN)) {
+      // Just use that PHI instead then.
+      ++NumPHICSEs;
+      return replaceInstUsesWith(PN, &IdenticalPN);
+    }
+
+    // Look for the following pattern and do PHI CSE to clean up the
+    // redundant %phi. Here %phi, %1 and %phi.next perform the same
+    // functionality as %identicalPhi and hence %phi can be eliminated.
+    //
+    // BB1:
+    //   %identicalPhi = phi [ X, %BB0 ], [ %identicalPhi.next, %BB1 ]
+    //   %phi = phi [ X, %BB0 ], [ %phi.next, %BB1 ]
+    //   ...
+    //   %identicalPhi.next = select %cmp, %val, %identicalPhi
+    //                      (or select %cmp, %identicalPhi, %val)
+    //   %1 = select %cmp2, %identicalPhi, %phi
+    //   %phi.next = select %cmp, %val, %1
+    //             (or select %cmp, %1, %val)
+    //
+    // Prove that %phi and %identicalPhi are the same by induction:
+    //
+    // Base case: Both %phi and %identicalPhi are equal on entry to the loop.
+    // Inductive case:
+    // Suppose %phi and %identicalPhi are equal at iteration i.
+    // We look at their values at iteration i+1 which are %phi.next and
+    // %identicalPhi.next. They would have become different only when %cmp is
+    // false and the corresponding values %1 and %identicalPhi differ
+    // (similar reason for the other "or" case in the bracket).
+    //
+    // The only condition when %1 and %identicalPh could differ is when %cmp2
+    // is false and %1 is %phi, which contradicts our inductive hypothesis
+    // that %phi and %identicalPhi are equal. Thus %phi and %identicalPhi are
+    // always equal at iteration i+1.
+
+    if (PN.getNumIncomingValues() == 2) {
+      unsigned DiffVals = 0;
+      BasicBlock *DiffValBB = nullptr;
+      // Check that only the backedge incoming value is different.
+      for (unsigned i = 0; i < 2; i++) {
+        BasicBlock *PredBB = PN.getIncomingBlock(i);
+        if (PN.getIncomingValueForBlock(PredBB) !=
+            IdenticalPN.getIncomingValueForBlock(PredBB)) {
+          DiffVals++;
+          DiffValBB = PredBB;
+        }
+      }
+
+      if (DiffVals != 1)
+        continue;
+      // Now check that the backedge incoming values are two select
+      // instructions that are in the same BB, and have the same condition.
+      // Either their true values are the same, or their false values are
+      // the same.
+      auto *Val = PN.getIncomingValueForBlock(DiffValBB);
+      auto *IdenticalVal = IdenticalPN.getIncomingValueForBlock(DiffValBB);
+      if (!isa<SelectInst>(Val) || !isa<SelectInst>(IdenticalVal))
+        continue;
+
+      auto *SI = cast<SelectInst>(Val);
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dtcxzyw wrote:

Use `dyn_cast` instead of `isa + cast`.

https://github.com/llvm/llvm-project/pull/163453